1. Field of the Invention
This invention concerns a thermoelectric module for generating electric power by utilizing the temperature difference and, vice versa, generating temperature difference in accordance with electric power applied.
2. Description of the Related Art
The thermoelectric module comprises a combination of P-type and N-type thermoelectric elements that utilize thermoelectric effects such as the Thomson effect, Peltier effect and Seebeck effect, and includes thermo-couples or electronic cooling elements. Since the thermoelectric module is simple in structure, easy to handle, and can maintain stable characteristics, it has been noted for the use in a wide range of applications. Particularly, as an electronic cooling element, since it can perform local cooling or precision temperature control near the room temperature. Research and development have been progressed generally for using a thermoelectric module for temperature control of opto-electronics devices or semiconductor lasers, as well as for small-sized refrigerators.
The figure of merit Z representing the performance of thermoelectric materials is expressed by the following equation using electrical resistivity xcfx81, thermal conductivity xcexa, and Seebeck coefficient xcex1:
z=xcex12/xcfx81xcexaxe2x80x83xe2x80x83(1)
In the equation, the Seebeck coefficient a takes a positive value in the P-type materials and a negative value in the N-type materials. A larger figure of merit Z is desired for the thermoelectric materials.
Further, the maximum value xcex7max for the conversion efficiency of the thermoelectric materials is represented by the following equation.                               η          max                =                                            Δ              ⁢                              xe2x80x83                            ⁢              T                                      T              h                                ⁢                                    M              -              1                                      M              +                                                T                  c                                                  T                  h                                                                                        (        2        )            
where Th is a temperature on the high temperature side, Tc is a temperature on the lower temperature side and the difference of the temperature xcex94T is represented by the following equation:
xcex94T=Thxe2x88x92Tcxe2x80x83xe2x80x83(3)
Further M is defined by the following equations (4) to (7).
xe2x80x83M={square root over ({overscore (ZT)})}+1xe2x80x83xe2x80x83(4)
{overscore (ZT)}={overscore (Z)}xc3x97{overscore (T)}xe2x80x83xe2x80x83(5)
                              Z          _                =                                            ∫                              T                c                                            T                h                                      ⁢                          Z              ⁢                              xe2x80x83                            ⁢                              ⅆ                T                                                          Δ            ⁢                          xe2x80x83                        ⁢            T                                              (        6        )                                          T          _                =                                            T              c                        +                          T              h                                2                                    (        7        )            
As materials used for the P-type thermoelectric element, Mnxe2x80x94Si series materials disclosed in the paper article of V. K. Zaitsev (Thermoelectric Engineering Handbook, p299-309, published from CRC in 1995) have high conversion efficiency of about 11% at 650xc2x0 C. Further, the report by Matsubara (Proceedings of Thermoelectric Conversion Symposium ""97 (TEC ""97), Thermoelectric Conversion Study Group, Jul. 25, 1997) discloses the use of compounds having the skutterudite structure as the material for the P-type element. The term xe2x80x9cskutteruditexe2x80x9d is derived from minerals CoAs3 produced in Skutterud, that is, the name of a district in Norway. The literature describes that CoSb3, RhSb3 and IrSb3, which have the skutterudite structure, are P-type semiconductors having inherent band structures and carrier transport characteristics. CoSb3, RhSb3 and IrSb3 also have a feature in that the hole mobility is large, such as from 2000 to 3000 cm2/Vs at a room temperature. P-type Zn4Sb4 used so far is fragile, and its usable temperature is low. Ce(FeCo)4Sb12 is also fragile and tends to be oxidized in an air atmosphere at 500xc2x0 C. or higher. Further, the SiGe series and FeSi series materials involve a problem of low figure of merit.
On the other hand, in order to construct an excellent thermoelectric module, not only excellent P-type thermoelectric element but also excellent N-type thermoelectric element are required. Heretofore, the Mgxe2x80x94Sixe2x80x94Sn series, SiGe series, FeSi series, Pbxe2x80x94Te series or Pbxe2x80x94Se series materials have been used as the N-type thermoelectric element.
However, the N-type Mg2 (Sixe2x80x94Sn) tends to be oxidized in an air atmosphere at a temperature of 500xc2x0 C. or higher. The Pbxe2x80x94Te series or Pbxe2x80x94Se series materials have a worry of giving undesirable effects on environments. Further, the SiGe series and the FeSi series materials involve a problem of low figure of merit
In view of the above, this invention intends to provide a thermoelectric module using N-type thermoelectric elements having excellent characteristics in an air atmosphere even when the temperature rises to a medium-to-high temperature region of about 500xc2x0 C. and, further, to improve the conversion efficiency of a thermoelectric module by the combination of an excellent P-type thermoelectric material and an excellent n-type thermoelectric material.
For solving the foregoing problem, this invention provides a thermoelectric module comprising N-type thermoelectric elements each containing a compound having a skutterudite structure and P-type thermoelectric elements each containing an Mnxe2x80x94Si series compound and connected directly or through a metallic component to the N-type thermoelectric elements.
This invention also provides a thermoelectric module comprising an insulator in which a plurality of openings are formed in a lattice configuration, N-type thermoelectric elements each containing a compound having a skutterudite structure and disposed in a first opening of the insulator, P-type thermoelectric elements each containing an Mnxe2x80x94Si series compound and disposed in a second opening of the insulator, and a metal member for connecting the N-type thermoelectric element and the P-type thermoelectric element.
According to this invention, since a compound having a skutterudite structure is used for the N-type thermoelectric element, excellent characteristics can be obtained in an air atmosphere even at a temperature of around 500xc2x0 C. As the material for the N-type thermoelectric element, for example, an Coxe2x80x94Sb series compound can be used. Further, when the Mnxe2x80x94Si series compound is used as the material for the P-type thermoelectric element, the conversion efficiency of a thermoelectric module can be improved by the combination of the excellent P-type thermoelectric material and the excellent N-type thermoelectric material.